[PATCH 3/4] gfp: mm: introduce __GFP_NOINIT

[Kees Cook]

On Wed, May 8, 2019 at 8:38 AM Alexander Potapenko <glider at google.com> wrote:
> When passed to an allocator (either pagealloc or SL[AOU]B), __GFP_NOINIT
> tells it to not initialize the requested memory if the init_on_alloc
> boot option is enabled. This can be useful in the cases newly allocated
> memory is going to be initialized by the caller right away.
>
> __GFP_NOINIT doesn't affect init_on_free behavior, except for SLOB,
> where init_on_free implies init_on_alloc.
>
> __GFP_NOINIT basically defeats the hardening against information leaks
> provided by init_on_alloc, so one should use it with caution.
>
> This patch also adds __GFP_NOINIT to alloc_pages() calls in SL[AOU]B.
> Doing so is safe, because the heap allocators initialize the pages they
> receive before passing memory to the callers.
>
> Slowdown for the initialization features compared to init_on_free=0,
> init_on_alloc=0:
>
> hackbench, init_on_free=1:  +6.84% sys time (st.err 0.74%)
> hackbench, init_on_alloc=1: +7.25% sys time (st.err 0.72%)
>
> Linux build with -j12, init_on_free=1:  +8.52% wall time (st.err 0.42%)
> Linux build with -j12, init_on_free=1:  +24.31% sys time (st.err 0.47%)
> Linux build with -j12, init_on_alloc=1: -0.16% wall time (st.err 0.40%)
> Linux build with -j12, init_on_alloc=1: +1.24% sys time (st.err 0.39%)
>
> The slowdown for init_on_free=0, init_on_alloc=0 compared to the
> baseline is within the standard error.
>
> Signed-off-by: Alexander Potapenko <glider at google.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Masahiro Yamada <yamada.masahiro at socionext.com>
> Cc: James Morris <jmorris at namei.org>
> Cc: "Serge E. Hallyn" <serge at hallyn.com>
> Cc: Nick Desaulniers <ndesaulniers at google.com>
> Cc: Kostya Serebryany <kcc at google.com>
> Cc: Dmitry Vyukov <dvyukov at google.com>
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Sandeep Patil <sspatil at android.com>
> Cc: Laura Abbott <labbott at redhat.com>
> Cc: Randy Dunlap <rdunlap at infradead.org>
> Cc: Jann Horn <jannh at google.com>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: linux-mm at kvack.org
> Cc: linux-security-module at vger.kernel.org
> Cc: kernel-hardening at lists.openwall.com
> ---
>  include/linux/gfp.h | 6 +++++-
>  include/linux/mm.h  | 2 +-
>  kernel/kexec_core.c | 2 +-
>  mm/slab.c           | 2 +-
>  mm/slob.c           | 3 ++-
>  mm/slub.c           | 1 +
>  6 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index fdab7de7490d..66d7f5604fe2 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -44,6 +44,7 @@ struct vm_area_struct;
>  #else
>  #define ___GFP_NOLOCKDEP       0
>  #endif
> +#define ___GFP_NOINIT          0x1000000u

I mentioned this in the other patch, but I think this needs to be
moved ahead of GFP_NOLOCKDEP and adjust the values for GFP_NOLOCKDEP
and to leave the IS_ENABLED() test in __GFP_BITS_SHIFT alone.

>  /* If the above are modified, __GFP_BITS_SHIFT may need updating */
>
>  /*
> @@ -208,16 +209,19 @@ struct vm_area_struct;
>   * %__GFP_COMP address compound page metadata.
>   *
>   * %__GFP_ZERO returns a zeroed page on success.
> + *
> + * %__GFP_NOINIT requests non-initialized memory from the underlying allocator.
>   */
>  #define __GFP_NOWARN   ((__force gfp_t)___GFP_NOWARN)
>  #define __GFP_COMP     ((__force gfp_t)___GFP_COMP)
>  #define __GFP_ZERO     ((__force gfp_t)___GFP_ZERO)
> +#define __GFP_NOINIT   ((__force gfp_t)___GFP_NOINIT)
>
>  /* Disable lockdep for GFP context tracking */
>  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>
>  /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (23 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (25)

AIUI, this will break non-CONFIG_LOCKDEP kernels: it should just be:

-#define __GFP_BITS_SHIFT (23 + IS_ENABLED(CONFIG_LOCKDEP))
+#define __GFP_BITS_SHIFT (24 + IS_ENABLED(CONFIG_LOCKDEP))

>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>
>  /**
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ee1a1092679c..8ab152750eb4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2618,7 +2618,7 @@ DECLARE_STATIC_KEY_FALSE(init_on_alloc);
>  static inline bool want_init_on_alloc(gfp_t flags)
>  {
>         if (static_branch_unlikely(&init_on_alloc))
> -               return true;
> +               return !(flags & __GFP_NOINIT);
>         return flags & __GFP_ZERO;

What do you think about renaming __GFP_NOINIT to __GFP_NO_AUTOINIT or something?

Regardless, yes, this is nice.

-- 
Kees Cook